Do Custom normals work in Animation Nodes?
You can get access to loops normals using the following expression:

[loop.normal for loop in [obj.data.calc_normals_split(), obj][1].data.loops]


Which calculates the split normals data and returns the normals. To set vertex normals to a mesh, you can use the setVertexNormals method, which take a single vector list representing the normals.